Type 1 diabetes (T1D) is largely caused by effector T cell mediated infiltration and autoimmune destruction of the pancreatic islets. Islet-specific T cells must enter the islets to induce pathology. Therefore, blocking T cells from infiltrating the islets could be a treatment strategy to prevent beta cell destruction. However, little is known about the process through which T cells infiltrate the pancreatic islets by traversing the vascular endothelium. To prevent this pathogenic infiltration of the islets we must first understand the mechanisms by which T cells infiltrate the islets. The goal of this proposal is to test cytoskeleta proteins as novel targets to inhibit T cell entry into the islets and to identify the stage of extravasation at which they inhibit islet entry. Leukocyte entry into tissues is a major regulatory point of immune surveillance and function which plays a critical role in autoimmune tissue destruction. To infiltrate tissues, T cells must exit the blood by extravasating through the vascular endothelium, a process called trans-endothelial migration (TEM). To extravasate, T cells must adhere to the vascular wall, change shape, search for permissive sites of entry, extend membrane protrusions such as filopodia, invasive podosomes, and lamellipodia, and generate force to push through the vascular wall. Little is known about how the T cell cytoskeleton regulates the shape changes necessary for TEM. Our preliminary data show that TEM is regulated by members of the Formin family of cytoskeletal effector molecules, which promote formation of elongated protrusions such as filopodia and invasive podosomes by mediating linear actin polymerization. These structures can serve as sensory protrusions and have been suggested to play a role in TEM. We have established a system for intra-vital imaging of the pancreatic islets that allows us to analyze T cell TEM into the islets. Our data show that TEM through the islet vasculature takes approximately five times longer than the process of TEM into the lymph nodes during normal immune surveillance. Because of this difference, we hypothesize that the Formin family of cytoskeletal effector molecules have profound effects on TEM in the more restrictive islet vasculature as compared to lymph nodes, thereby inhibiting T cell infiltration into the islets more dramatically. To determine how these cytoskeletal molecules function in T cell entry into the islets we propose to determine the effects of inhibition of Formin family proteins on (1) T cell entry into the islets, (2) completing the stes of TEM into the islets, and (3) prevention of type 1 diabetes induction. Our results will assess the possibility of targeting cytoskeletal molecules to inhibit T cell entry into the islets in T1D without preventing the normal immune response, thereby representing a novel therapeutic approach.